A one-way valve of the kind outlined above is already known from patent application Ser. No. WO 02/087993. A valve of the kind referred to is used in particular in conjunction with cooking and packaging of ready-to-eat dishes, in a manner ensuring that the valve functions both when the food is being cooked and when it is finally heated preparatory to consumption. When the heating process in conjunction with the cooking of the food is stopped, the valve should close, and this could be achieved for example purely mechanically by means outside the package, pressing said layer against the package material. Alternatively, valve closing could be achieved by choosing an adhesive layer of a kind that resumes its position on top of the aperture in the package material automatically and in doing so closes the valve. On account of the considerable flow of hot vapour through the valve bubbles may form between the adhesive layer and the package material in the areas where said adhesive layer has separated from the package material (see FIG. 1) when the valve closes following the initial heating in conjunction with the packaging of the food. In some cases, a series of bubbles may form in this manner, and these bubbles may form a channel running from the aperture formed in the package material and up to one of the edges of the adhesive layer.